The principal goal of this unit continues to be the detailed biological and biochemical characterization of the human papovaviruses, BKV and JCV. During the past six years, most of the research efforts of this unit were directed toward studies with BKV since this virus could easily be grown in cell cultures of various types. JCV, on the other hand, has been difficult to propagate in vitro, and therefore has remained a relatively unknown virus. During the past year, however, a major advance was made when we were able to grow JCV in two different readily available cells, human amnion and human embryonic kidney cells. This will now enable us to conduct detailed investigations on the biology of this important virus. Studies are continuing on the mechanism of persistent infection of human fetal brain cells by BKV. It has been established that viral genomes exist in episomal form in these cells, and that rare cells in the culture spontaneously undergo lytic cycles to produce infectious progeny. This type of virus-cell interaction may explain how these papovaviruses persist indefinitely in the human host.